Optical filters are devices for selecting at least one optical frequency band, called a passband, out of an optical frequency spectrum of an optical signal. A central frequency of the passband of a tunable optical filter is adjustable, depending upon a control parameter common to a particular filter type. For example, for a bulk optic tunable filter, the control parameter can be a filter tilt or a clocking (rotation) angle with respect to an incoming optical beam. For an optical waveguide based tunable filter such as tunable Mach-Zehnder (MZ) interferometer, the control parameter can be an electrical signal applied to a localized heater that changes optical path length of one of its arms, which effectively tunes the MZ interferometer.
Tuning range, spectral selectivity, and a level of cross-talk suppression are very important parameters of tunable optical filters. A wide tuning range allows a wide range of optical frequencies to be accessed and selected by a tunable filter. The spectral selectivity relates to an ability of the filter to select a narrow frequency band of a broadband optical signal. Herein, the term “narrow” means small as compared to a value of the central frequency of the optical signal being filtered, for example 1% of the central frequency or less. Finally, the crosstalk suppression is an ability of the filter to suppress optical signals at any other frequency than the frequency of the signal being selected.
In an optical communications network optical signals, having a plurality of optical channels at individual optical frequencies or wavelengths called optical frequency channels or wavelength channels, are transmitted from one location to another, typically through a length of an optical fiber. Optical frequency channels can be combined for transmission through a single optical fiber, whereby the transmission capacity of the optical fiber increases many times. Since the optical frequency channels can be amplified simultaneously in a single optical amplifier, the transmission distances are increased, while the associated transmission costs are considerably reduced.
Tunable optical filters are used in optical communications networks for selecting at least one optical frequency channel out of a plurality of channels comprising an optical communications signal. Tunable optical filters are also used for system performance monitoring purposes, e.g. for performing a spectral measurement of the entire optical communications signal, including measuring optical noise levels between the neighboring frequency channels. The tunability of the filter allows any optical frequency component within the tuning range of the filter to be selected for subsequent detection and/or signal level measurement. It is very important that such a tunable filter have an excellent crosstalk suppression. A poor crosstalk suppression leads to undesired “leaking” of the optical channels being suppressed, which impairs the signal level measurements and/or detection and decoding of the selected signal.
U.S. Pat. No. 5,596,661 entitled “Monolithic Optical Waveguide Filters based on Fourier Expansion”, issued to Henry et al. of Lucent Technologies and incorporated herein by reference, teaches a planar lightwave circuit (PLC) optical filter having a chain of optical couplers linked by different delays with a transfer function equal to the sum of the contribution from each optical path, with each contribution forming a term in a Fourier series whose sum forms the optical output. Detrimentally, the optical filter of Henry et al. is not tunable.
U.S. Pat. No. 6,208,780 entitled “System and Method for Optical Monitoring”, issued to Li et al. of Lucent Technologies and incorporated herein by reference, teaches a tunable optical filter on a PLC chip using cascaded unbalanced MZ interferometers. In the tunable filter of Li et al., successive MZ stages have twice the free spectral range (FSR) as the previous MZ stages, thereby providing a narrowband optical filter having a wide tuning range.
With reference to FIG. 1, a prior-art tunable filter 10 of Li et al. is shown, having cascaded Mach-Zehnder (MZ) interferometers MZ1, MZ2, . . . , MZN connected in series. An optical signal 12 is applied to the first interferometer MZ1, the signal 12 exiting the filter 10 at an output waveguide of the last interferometer MZN. Each of the interferometers MZ1 . . . MZN has two branches, a non-adjustable branch 14 and an adjustable branch 16. Detrimentally, the tunable optical filter 10 requires many MZ stages, including stages that have to be repeated, to achieve a satisfactory crosstalk suppression.
European patent EP1492260A1 entitled “Optical ADD-DROP Multiplexer for WDM Systems”, issued to Crognale et al. and incorporated herein by reference, teaches a device that allows, by using selection and blocking functions of optical filtering and optical interleaving, to extract a single optical frequency channel from an optical communications signal and to insert another optical frequency channel at the same optical frequency instead of the one extracted. The device of Crognale et al. uses selective tunable filters to add or drop optical frequency channels. Detrimentally, it also requires many components such as blocking tunable filters, 1×N splitters, switches, and interleavers, which complicates the device and increases its size and cost.
An object of the present invention is to overcome the shortcomings of the prior art by providing a tunable optical filter on a single PLC chip, which combines narrowband spectral performance with wide tuning range and a high level of crosstalk suppression. Advantageously, a tunable optical filter of the present invention has no moving parts while having a switchable spectral resolution for operation at various optical frequency channel spacings. Further, advantageously, a tunable optical filter of the present invention is small enough to be placed within a single standard hot-pluggable XFP package.